The X-Seed 4000 is the tallest buildingever fully envisioned, meaning that the designs for the building have been completed. The idea was initially created and developed by Peter Neville. Its proposed 4-kilometre (2.5 mi) height, 6-kilometre (3.7 mi) wide sea-base, and 800-floor capacity could accommodate 500,000 to 1,000,000 inhabitants. This structure would be composed of over 3,000,000 tons of reinforced steel.

It was designed for Tokyo, Japan by the Taisei Corporation in 1995 as a futuristic environment combining ultra-modern living and interaction with nature. Methods of transportation within the X-seed would most likely include MagLev trains.

The X-Seed 4000 “is never meant to be built,” says Georges Binder, managing director of Buildings & Data, a firm which compiles data banks on buildings worldwide. “The purpose of the plan was to earn some recognition for the firm, and it worked.”

Unlike conventional skyscrapers, the X-Seed 4000 would be required to actively protect its occupants from considerable air pressure gradations and weather fluctuations along its massive elevation. Its design calls for the use of solar power to maintain internal environmental conditions. Also, the proposed area is situated on the Pacific Ring of Fire, which is the most active volcano range in the world, so the X-Seed 4000 would be subject to tsunamis and earthquakes. The Shimizu Mega-City Pyramid (proposed in 2007, also planned for Tokyo, Japan) faces most of the same problems.

A sea-based location and a Mount Fuji shape are some of this building’s other major design features — the real Mount Fuji is land-based and is 3,776 m (2.35 miles) high, 224 m shorter than the X-Seed 4000. The X-Seed 4000 is projected to be twice the height of the Shimizu Mega-City Pyramid at 2,004 m. Other projects that may be in the top five man made structures are the Ultima Tower (3,218 m), Dubai City Tower(2,400 m) and the Bionic Tower (1,228 m) in either Hong Kong or Shanghai.

Some estimate that the cost to construct the X-Seed 4000 may be somewhere between US$300–900 billion, in 2006 dollars ($353.00–$1.06 thousand billion in 2016).